Work has focused on the interaction between the primate malaria Plasmodium cynomolgi and the mosquito Anopheles gambiae, with special attention to factors which cause the normal process of mosquito-parasite infection to fail. Two lines of An. gambiae have been selected which are (respectively) refractory and susceptible to infection with P. cynomolgi. Although the lines are not yet pure, preliminary crosses suggest that susceptibility is dominant and inheritance may be monofactorial. Examination of the susceptibility of these two lines to other species of malaria has revealed the same refractoriness/susceptibility pattern with P. knowlesi and P. berghei. Although both lines are susceptible to infection with the human malarias P. vivax and P. falciparum, the refractory line does manifest consistently reduced susceptibility when compared with the non-selected and susceptible lines.